Theory And Data
     Physics  is  supposed  to  be  an  empirical 
      theory,  i.e.  a  theory  that  can  be  tested 
      against empirical data.
     Fundamental physics is supposed to provide 
      a complete physical ontology for the world.
     Therefore, either the data against which a 
      fundamental physical theory is tested is not 
      itself physical, or else it is somehow to be 
      understood as contained in the ontology of 
      the theory itself.
                  “Empirical”
     Saying that a theory is tested against empirical 
      data  suggests  that  the  data  is  somehow 
      “empirical”.
     “Empirical” comes from the Greek “ἐμπειρία”, 
      which is usually translated as “experience”.
     One    might    then   conclude    that   either 
      experience is itself a physical thing, and hence 
      part of the fundamental physical ontology, or 
      else some extra principle is required to connect 
      claims  about  the  physical  ontology  to  the 
      experiential data.
                   Dilemma
     Both  options  here  seem  unpalatable.  One 
      requires  giving  a  physical  analysis  of 
      experience  itself,  and  hence  solving  the 
      mind-body  problem.  The  other  requires 
      some  sort  of  “bridge  principle”  from 
      physical claims to experiential claims, and 
      raises   the   question    of   both    the 
      metaphysical status and justification of the 
      bridge principle.
            Logical Empiricism
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      In  the  early  20   century,  the  school  of 
      logical empiricists tried to dress up the old 
      empiricist  account  of  meaning  found  in 
      Hume and Locke with the modern garb of 
      predicate  calculus.  Many  (but  not  all) 
      followed  the  principle  that  all  meaningful 
      claims must either be analytic (and hence 
      “empty”)  or  reducible  to  claims  about 
      experience,  such  as  a  particular  sort  of 
      visual  experience  (“red-spot-here-for-me-
      now”).
            Bridge Principles
     Since claims about conscious experience do 
      not follow directly from claims about, e.g., 
      the  behavior  of  atoms,  some  logical 
      empiricists who wanted to connect physical 
      claims to data needed bridge principles.